The present invention relates to microelectronic connection components and more specifically relates to microelectronic assemblies incorporating soldered connections and to components incorporating pads for soldering.
Soldered connections are used throughout the electronics industry to connect electronic components. Where the components to be connected include dielectric elements such as a rigid circuit panel or a flexible dielectric circuit panel with conductive metallic traces, the traces may be provided with enlarged regions, commonly referred to as xe2x80x9clandsxe2x80x9d or xe2x80x9cpadsxe2x80x9d. The traces may extend along a surface of the dielectric element. A further dielectric element, commonly referred to as a xe2x80x9csolder maskxe2x80x9d or xe2x80x9ccoverlayxe2x80x9d may be provided. The solder mask layer may be applied by laminating a preformed dielectric sheet to the surface of the dielectric element, or by forming the dielectric sheet from a curable liquid on the surface of the dielectric element. The solder mask has holes at spacings corresponding to the spacings of the pads. The solder mask coverlay closely overlies the trace-bearing surface of the panel and closely overlies the metallic traces, leaving all or part of each pad exposed at the corresponding hole in the solder mask. A mass of solder may be deposited on each pad, either by exposing the assembly to a liquid solder, as in a wave soldering or dip soldering process or, more typically, by applying solder performs commonly referred to as xe2x80x9csolder ballsxe2x80x9d onto the pads and heating the assembly to melt the solder. The molten solder wets the metal of the pads and forms a strong bond to the pads. The solder mask layer, which is not wettable by the solder confines the solder on the pads. In the absence of the solder mask layer, the molten solder could wet the metal in the traces extending away from the pads and hence could flow outwardly, along the traces. This would provide solder in undesired locations and displace the solder mass from its desired location, centered on the pad. Moreover the undesired solder flow can remove solder from the pads where it is required for forming the joints. The solder mask prevents this undesired flow.
After application of the solder masses, the component has solder masses protruding from the surface. A component such as a semiconductor chip package having an array of solder masses on a surface in a grid-like pattern is sometimes referred to as a xe2x80x9cball-grid arrayxe2x80x9d element. The use of ball-grid arrays in packages for microelectronic devices such as semiconductor chips is described for example, in the article xe2x80x9cTBGA Package Technology,xe2x80x9d IEEE Transactions on Components, Packaging and Manufacturing Technology, Part B, Vol. 17, No. 4, VP 564-568 by Andros and Hammer and in xe2x80x9cBall Grid Array Technology,xe2x80x9d Lau, J. H. ed, pp. 460-464. As described, for example, in commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266, a microelectronic component such as a semiconductor chip may include a set of pads in the form of terminals which may be mounted on a dielectric layer such as a flexible sheet. The pads or terminals may be connected to contacts on the chip by flexible leads and may be supported above the surface of the chip by a compliant layer such as an elastomer interposed between the terminals and the chip, typically between the dielectric layer and the chip. Masses of solder may be provided on the pads or terminals for connecting the assembly to a circuit board or other substrate having corresponding pads.
The component can be engaged with another component having a corresponding set of pads and, typically, also having similar solder mask. After engaging the protruding solder masses with the pads of the other component, the solder masses may be heated again to melt all or part of each solder mask and bond the solder masses to the pads of the other component. The resulting solder columns interconnect pads on both components with one another electronically and also form a mechanical connection between the components. In a variant of this process, each solder ball may include a core, typically formed from a conductive metal such as copper or nickel which does not melt at the temperatures used to melt the solder. Such a core is commonly referred to as a xe2x80x9csolid corexe2x80x9d. A xe2x80x9csolidxe2x80x9d core in this context may have one or more interior voids, or else may be entirely free of voids. The resulting solder joint includes the solid core embedded within an outer layer of solder. Alternatively, the protruding solder elements may be engaged with a small socket having holes and metallic resilient elements designed to accept and engage the solder elements in the holes. Sockets of this type are disclosed in certain embodiments of International Patent Publication WO 95/34106. Other sockets which can engage protruding solder or other elements are described in International Patent Publication WO 97/44859.
One particularly useful approach described in copending commonly assigned in U.S. patent application Ser. No. 411,472 filed Mar. 28, 1995 incorporates solid core solder assemblies in components with having pads supported on a compliant layer. In certain structures according to the ""472 application, the compliant layer allows the pads and hence the solid core solder assemblies to move when the component with the assemblies thereon is juxtaposed with another component. This allows the solid core solder assemblies to engage the pads on the opposite component despite some minor deviations from perfect planarity of the pads on the components. Moreover, the compliant layer in the assembly mechanically decouples the pads from movement of the underlying component caused by thermal expansion and contraction and thus reduces the mechanical stresses, such as fatigue on the solder assemblies. The mechanical decoupling also facilitates use of the component with a socket adapted to receive the solid core solder balls.
Some typical configurations for pads or lands are described for example in American National Standard ANSI/IPC-D-249, Design Standard for Flexible Single and Double Sided Printed Boards; in American National Standard ANSI/IPCD-275 of September 1991 entitled Design Standard for Rigid Printed Boards and Rigid Printed Board Assemblies, pp.62-68; in Design Guidelines for Surface Mount and Finexe2x80x94Pitch Technology, 2nd edition, 1996 by Vernon Solberg, pp.142-143; and in Fjelstad, An Engineer""s Guide to Flexible Circuit Technology (ElectroChemical Publications Limited 1997; ISBN 0901150347), pp.148-149.
Many common pads are round, circular bodies of conductive material. However, other shapes, such as hollow circles or squares of conductive material have been employed. Also, round and square lands with branches or ribs extending from them have been employed as illustrated in Standard ANSI/IPC-D-350, Revision D, July 1992 and in Natarajan et al., U.S. Pat. No. 5,519,580. Still other solder land configurations are disclosed in IBM Technical Disclosure Bulletin, Vol. 40, No. 6, pp.199-200 entitled xe2x80x9cBall Grid Array Solder Ball on Ball Grid Array Dimple Padxe2x80x9d (June 1997); in U.S. Pat. No. 5,400,220 and in U.S. Pat. No. 5,133,495.
Despite all of these efforts in the art however, further needs for improvement in microelectronic assemblies incorporating solder joints, and in method components for making such joints, remain.
One aspect of the present invention provides methods of fabricating microelectronic assemblies. A method in accordance with this aspect of the invention desirably includes the steps of providing a component including a base and one or more pads on a base. Each pad includes a plurality of line segments or strips of solder-wettable material such as a metal extending outwardly away from a center, the lines of solder-wettable material being bounded by non-solder wettable material which may be metallic or non-metallic. The method further includes the steps of providing a mass of a molten solder and a solid core on at least some the pads so that at each pad where the mass and core are provided, the molten solder contacts the core and also contacts the wettable lines of the solder pad and then cooling the solder, cores and pads to solidify the solder.
Methods according to this aspect of the invention incorporate the realization that pads having outwardly extending wettable strips provide certain unique benefits when used with solid core solder balls. In particular, the assembly which results when a solid core solder ball is reflowed on such a pad tends to have the solid core in the desired position, resting directly on the pad with little or no solder intervening between the core and the pad, and with the core well centered on the pad. Although the present invention is not limited by any theory of operation, it is believed that this action results from the patterns of a surface tension which arise when the wettable lines are provided. Regardless of the reasons for this phenomenon, provision of the solid core in the desired positions provides several benefits. First, nonuniformities in the height of the solid core solder balls caused by differences position of the core are minimized. This, in turn, facilitates engagement of the component having the solid core solder balls with pads or sockets on a mating component. Also, the enhanced centering of the cores provides solid core solder balls at predictable, horizontal locations corresponding to the horizontal locations of the pad centers on the base. For example, where the pads are disposed at a uniform center-to-center spacing or xe2x80x9cpitchxe2x80x9d, the solid core solder balls will also be at a uniform pitch. This in turn facilitates engagement of the solid core solder balls in multiple sockets of a socket element or engagement of the solid core solder balls with pads on an opposing component.
Moreover, placement of the cores at the preferred location touching the pad or very close to the pad minimizes the amount of solder disposed between the core and the pad. This, in turn, leaves more of the solder free to form generous fillets surrounding the juncture of the solder ball and the pad. This in turn enhances the assembly to resist fatigue stress. Also, the outwardly extending strips provide locally enlarged fillets at the various locations around the periphery of the joint between the solder ball and the pad, thereby further reinforcing the structure against mechanical stress. In particularly preferred arrangements, the cores project horizontally over the lines. For example, the cores may be spherical and the strips of solder-wettable material of each pad may extend radially inwardly so that the inner ends of the lines lie at a distance from the center of the pad which is smaller than the radius of the core associated with the pad.
Each pad desirably includes at least three lines of solder wettable material. Each pad may also include a central body, so that the strips join the central body and project outwardly from the central body. Alternatively, the pad may consist entirely of the strips. The strips may extend all the way inwardly to intersect one another at the center of the pad, or may stop at a small distance from the center so that the strips do not intersect one another.
Additional methods according to this aspect of the invention may include the step of further assembling the component to other elements. For example, the solid core solder balls projecting from the pads may be engaged in sockets of a multi-socket element. Alternately, the component with the solid core solder balls thereon may be juxtaposed with a substrate having additional pads thereon. Such engagement may be performed during the original step of providing the mass of molten solder and the solid core, so that the pads on the substrate are engaged with the molten solder before the solder first cools to bond the molten solder and the solid core to the pads of the component. Alternately, the component may be provided with the cooled and bonded solid core solder balls and then may be juxtaposed with the substrate having mating pads thereon and reheated to form bonds with the mating pads. In either case, the mating pads on the substrate may also include lines of solder-wettable material as aforesaid.
Methods according to further aspects of the invention include the steps of providing a component incorporating a base and one or more pads on the base, each such pad having a surface with a solder wettable area and a non-solder wettable area. Methods according to this aspect of the invention further include the steps of providing a mass of molten solder on each pad so that the molten solder on each pad to the wettable area of the pad surface and is wholly or partially confined by the non-wettable area of the pad surface and then cooling the pads and solder to solidify the solder. The non-wettable areas desirably act to prevent spreading of the solder as, for example, to prevent solder from spreading from the pad along a trace intersecting the pad. The non-wettable areas may include one or more non-wettable regions adjacent the periphery of the pad. For example, the non-wettable areas may include non-wettable areas at an intersection of the pad and a trace may also include a loop-like border of non-wettable surface encircling the wettable areas. In methods according to these embodiments of the invention, the non-wettable areas of the pads themselves provide solder confinement. Therefore, a separate solder mask is not required for confinement of the molten solder. Omission of the solder mask can provide significant savings in costs and complexity of the assembly. Moreover, there is no need to provide a solder mask in precise registration with the pads. This is of particular significance in assemblies incorporating small pads. In assemblies which include solder masks for confinement of the solder, misregistration with the solder mask and the pads effectively reduces the area of the pad available for solder bonding and can result in a bond having an undesirable shape susceptible to fatigue stresses in service. This possibility is effectively eliminated in preferred methods according to this aspect of the present invention. Even if a separate solder mask is used, the solder mask may have holes substantially larger than the pads, so that misregistration will not result in coverage of any of the pad by the solder mask.
In further methods according to this aspect of the invention, the wettable areas on the pads may incorporate a plurality of lines extending generally radially with respect to the center of each pad. Pads defining such lines may be used in conjunction with solid core solder balls to provide the benefits discussed above in connection with solid core solder ball joints, and can also be used in conjunction with other solder masses.
Still further aspects of the present invention provide components for forming solder connections. These components include a base and a plurality of pads on the base. Each pad includes a conductive, preferably metallic element having a top surface facing away the base. The pad has wettable and non-wettable surface areas on the top surface of the conductive element. The wettable area may include, for example, a plurality of lines extending radially with respect to the center of the pad. Alternatively or additionally, each pad may include a non-wettable area effective to confine solder on the wettable area as, for example, one or more non-wettable areas at a juncture of the pad and a trace or other metallic feature or a non-wettable area in a loop-like configuration encircling the wettable area, or both. The wettable area typically is defined by exposed region of the metallic element as, for example, exposed copper, gold or other pure metal. The non-wettable area may have a non-wettable pure metal or metal alloy at its surface as, for example, one or more metals selected from the group consisting of nickel, chromium, rhodium, osmium, and combinations thereof. Alternatively or additionally, the non-wettable area may include a metal compound such as an oxide or nitride of a metal. According to a further variant, the non-wettable areas of the pads may be provided by dielectric materials such as polymers selectively deposited on the pads by an electrophoretic or other process. Components according to this aspect of the invention and may be used in a process as discussed above.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings.